After the Great War between continents, human society was recluse underground. Tiny as ants, men started building small rooms underground. They decided to reconstruct their world next to the roots of the trees, from which they took all kinds of minerals, food and beverage. The roots traced the way towards juicy insects and water particles, but also towards the surface they had lost contact with.
The trees, apparently passive, reconfigured their roots and structure at a rhythm unknown for anyone else, with a silent music. In a time without beginning nor ending, different to that of their tiny human hosts, the trees transported them around the whole world, carrying with them their thirst, the air and the beasts. All the good things, towards human cities deeper underground.
Humans never ever came back up to see the sunlight, and instead fed the earth, the trees and the beasts when they perished.